Apparatus and method for maintaining immersion fluid in the gap under the  projection lens during wafer exchange in an immersion lithography machine

ABSTRACT

An apparatus and method maintain immersion fluid in the gap adjacent to the projection lens during the exchange of a work piece in a lithography machine. The apparatus and method include an optical assembly that projects an image onto a work piece and a stage assembly including a work piece table that supports the work piece adjacent to the optical assembly. An environmental system is provided to supply and remove an immersion fluid from the gap between the optical assembly and the work piece on the stage assembly. After exposure of the work piece is complete, an exchange system removes the work piece and replaces it with a second work piece. An immersion fluid containment system maintains the immersion liquid in the gap during removal of the first work piece and replacement with the second work piece.

RELATED APPLICATIONS

This is a Divisional of U.S. patent application Ser. No. 11/237,721filed Sep. 29, 2005, which in turn is a Continuation of InternationalApplication No. PCT/IB2004/001259 filed Mar. 17, 2004, which claims thebenefit of U.S. Provisional Application No. 60/462,499 filed on Apr. 11,2003. The entire disclosures of the prior applications are incorporatedherein by reference in their entireties.

BACKGROUND

Lithography systems are commonly used to transfer images from a reticleonto a semiconductor wafer during semiconductor processing. A typicallithography system includes an optical assembly, a reticle stage forholding a reticle defining a pattern, a wafer stage assembly thatpositions a semiconductor wafer, and a measurement system that preciselymonitors the position of the reticle and the wafer. During operation, animage defined by the reticle is projected by the optical assembly ontothe wafer. The projected image is typically the size of one or more dieon the wafer. After an exposure, the wafer stage assembly moves thewafer and then another exposure takes place. This process is repeateduntil all the die on the wafer are exposed. The wafer is then removedand a new wafer is exchanged in its place.

Immersion lithography systems utilize a layer of immersion fluid thatcompletely fills a gap between the optical assembly and the wafer duringthe exposure of the wafer. The optic properties of the immersion fluid,along with the optical assembly, allow the projection of smaller featuresizes than is currently possible using standard optical lithography. Forexample, immersion lithography is currently being considered for nextgeneration semiconductor technologies including 65 nanometers, 45nanometers, and beyond. Immersion lithography therefore represents asignificant technological breakthrough that will likely enable thecontinued use of optical lithography for the foreseeable future.

After a wafer is exposed, it is removed and exchanged with a new wafer.As currently contemplated in immersion systems, the immersion fluidwould be removed from the gap and then replenished after the wafer isexchanged. More specifically, when a wafer is to be exchanged, the fluidsupply to the gap is turned off, the fluid is removed from the gap(i.e., by vacuum), the old wafer is removed, a new wafer is aligned andplaced under the optical assembly, and then the gap is re-filled withfresh immersion fluid. Once all of the above steps are complete,exposure of the new wafer can begin.

Wafer exchange with immersion lithography as described above isproblematic for a number of reasons. The repeated filling and drainingof the gap may cause variations in the immersion fluid and may causebubbles to form within the immersion fluid. Bubbles and the unsteadyfluid may interfere with the projection of the image on the reticle ontothe wafer, thereby reducing yields. The overall process also involvesmany steps and is time consuming, which reduces the overall throughputof the machine.

An apparatus and method for maintaining immersion fluid in the gapadjacent to the projection lens when the wafer stage moves away from theprojection lens, for example during wafer exchange, is therefore needed.

SUMMARY

An apparatus and method maintain immersion fluid in the gap adjacent tothe projection lens in a lithography machine. The apparatus and methodinclude an optical assembly that projects an image onto a work piece anda stage assembly including a work piece table that supports the workpiece adjacent to the optical assembly. An environmental system isprovided to supply and remove an immersion fluid from the gap. Afterexposure of the work piece is complete, an exchange system removes thework piece and replaces it with a second work piece. An immersion fluidcontainment system is provided to maintain the immersion fluid in thegap when the work piece table moves away from the projection lens. Thegap therefore does not have to be refilled with immersion fluid when thefirst work piece is replaced with a second work piece.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the followingdrawings of exemplary embodiments in which like reference numeralsdesignate like elements, and in which:

FIG. 1 is an illustration of an immersion lithography machine havingfeatures of the invention;

FIG. 2 is a cross section of an immersion lithography machine accordingto one embodiment of the invention;

FIGS. 3A and 3B are a cross section and a top down view of an immersionlithography machine according to another embodiment of the invention;

FIGS. 4A and 4B are cross section views of an immersion lithographymachine according to another embodiment of the invention;

FIGS. 5A and 5B are top down views of two different twin wafer stagesaccording to other embodiments of the invention;

FIG. 6A is a top down view of a twin stage lithography machine accordingto another embodiment of the invention;

FIGS. 6B-6E are a series of diagrams illustrating a wafer exchangeaccording to the invention;

FIG. 7A is a flow chart that outlines a process for manufacturing a workpiece in accordance with the invention; and

FIG. 7B is a flow chart that outlines work piece processing in moredetail.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 is a schematic illustration of a lithography machine 10 havingfeatures of the invention. The lithography machine 10 includes a frame12, an illumination system 14 (irradiation apparatus), an opticalassembly 16, a reticle stage assembly 18, a work piece stage assembly20, a measurement system 22, a control system 24, and a fluidenvironmental system 26. The design of the components of the lithographymachine 10 can be varied to suit the design requirements of thelithography machine 10.

In one embodiment, the lithography machine 10 is used to transfer apattern (not shown) of an integrated circuit from a reticle 28 onto asemiconductor wafer 30 (illustrated in phantom). The lithography machine10 mounts to a mounting base 32, e.g., the ground, a base, or floor orsome other supporting structure.

In various embodiments of the invention, the lithography machine 10 canbe used as a scanning type photolithography system that exposes thepattern from the reticle 28 onto the wafer 30 with the reticle 28 andthe wafer 30 moving synchronously. In a scanning type lithographicmachine, the reticle 28 is moved perpendicularly to an optical axis ofthe optical assembly 16 by the reticle stage assembly 18, and the wafer30 is moved perpendicularly to the optical axis of the optical assembly16 by the wafer stage assembly 20. Scanning of the reticle 28 and thewafer 30 occurs while the reticle 28 and the wafer 30 are movingsynchronously.

Alternatively, the lithography machine 10 can be a step-and-repeat typephotolithography system that exposes the reticle 28 while the reticle 28and the wafer 30 are stationary. In the step and repeat process, thewafer 30 is in a constant position relative to the reticle 28 and theoptical assembly 16 during the exposure of an individual field.Subsequently, between consecutive exposure steps, the wafer 30 isconsecutively moved with the wafer stage assembly 20 perpendicularly tothe optical axis of the optical assembly 16 so that the next field ofthe wafer 30 is brought into position relative to the optical assembly16 and the reticle 28 for exposure. Following this process, the imageson the reticle 28 are sequentially exposed onto the fields of the wafer30, and then the next field of the wafer 30 is brought into positionrelative to the optical assembly 16 and the reticle 28.

However, the use of the lithography machine 10 provided herein is notnecessarily limited to a photolithography for semiconductormanufacturing. The lithography machine 10, for example, can be used asan LCD photolithography system that exposes a liquid crystal displaywork piece pattern onto a rectangular glass plate or a photolithographysystem for manufacturing a thin film magnetic head. Accordingly, theterm “work piece” is generically used herein to refer to any device thatmay be patterned using lithography, such as but not limited to wafers orLCD substrates.

The apparatus frame 12 supports the components of the lithographymachine 10. The apparatus frame 12 illustrated in FIG. 1 supports thereticle stage assembly 18, the wafer stage assembly 20, the opticalassembly 16 and the illumination system 14 above the mounting base 32.

The illumination system 14 includes an illumination source 34 and anillumination optical assembly 36. The illumination source 34 emits abeam (irradiation) of light energy. The illumination optical assembly 36guides the beam of light energy from the illumination source 34 to theoptical assembly 16. The beam illuminates selectively different portionsof the reticle 28 and exposes the wafer 30. In FIG. 1, the illuminationsource 34 is illustrated as being supported above the reticle stageassembly 18. Typically, however, the illumination source 34 is securedto one of the sides of the apparatus frame 12 and the energy beam fromthe illumination source 34 is directed to above the reticle stageassembly 18 with the illumination optical assembly 36.

The illumination source 34 can be a g-line source (436 nm), an i-linesource (365 nm), a KrF excimer laser (248 nm), an ArF excimer laser (193nm) or a F₂ laser (157 nm). Alternatively, the illumination source 34can generate an x-ray.

The optical assembly 16 projects and/or focuses the light passingthrough the reticle 28 to the wafer 30. Depending upon the design of thelithography machine 10, the optical assembly 16 can magnify or reducethe image illuminated on the reticle 28. The optical assembly 16 neednot be limited to a reduction system. It could also be a 1× or greatermagnification system.

Also, with an exposure work piece that employs vacuum ultra-violetradiation (VUV) of wavelength 200 nm or lower, use of a catadioptrictype optical system can be considered. Examples of a catadioptric typeof optical system are disclosed in Japanese Laid-Open Patent ApplicationPublication No. 8-171054 and its counterpart U.S. Pat. No. 5,668,672, aswell as Japanese Laid-Open Patent Publication No. 10-20195 and itscounterpart U.S. Pat. No. 5,835,275. In these cases, the reflectingoptical system can be a catadioptric optical system incorporating a beamsplitter and concave mirror. Japanese Laid-Open Patent ApplicationPublication No. 8-334695 and its counterpart U.S. Pat. No. 5,689,377 aswell as Japanese Laid-Open Patent Application Publication No. 10-3039and its counterpart U.S. Pat. No. 873,605 (Application Date: Jun. 12,1997) also use a reflecting-refracting type of optical systemincorporating a concave mirror, etc., but without a beam splitter, andalso can be employed with this invention. The disclosures of theabove-mentioned U.S. patents and applications, as well as the JapaneseLaid-Open patent application publications are incorporated herein byreference in their entireties.

The reticle stage assembly 18 holds and positions the reticle 28relative to the optical assembly 16 and the wafer 30. In one embodiment,the reticle stage assembly 18 includes a reticle stage 38 that retainsthe reticle 28 and a reticle stage mover assembly 40 that moves andpositions the reticle stage 38 and reticle 28.

Each stage mover assembly 40, 44 can move the respective stage 38, 42with three degrees of freedom, less than three degrees of freedom, ormore than three degrees of freedom. For example, in alternativeembodiments, each stage mover assembly 40, 44 can move the respectivestage 38, 42 with one, two, three, four, five or six degrees of freedom.The reticle stage mover assembly 40 and the work piece stage moverassembly 44 can each include one or more movers, such as rotary motors,voice coil motors, linear motors utilizing a Lorentz force to generatedrive force, electromagnetic movers, planar motors, or some other forcemovers.

In photolithography systems, when linear motors (see U.S. Pat. Nos.5,623,853 or 5,528,118 which are incorporated by reference herein intheir entireties) are used in the wafer stage assembly or the reticlestage assembly, the linear motors can be either an air levitation typeemploying air bearings or a magnetic levitation type using Lorentz forceor reactance force. Additionally, the stage could move along a guide, orit could be a guideless type stage that uses no guide.

Alternatively, one of the stages could be driven by a planar motor,which drives the stage by an electromagnetic force generated by a magnetunit having two-dimensionally arranged magnets and an armature coil unithaving two-dimensionally arranged coils in facing positions. With thistype of driving system, either the magnet unit or the armature coil unitis connected to the stage base and the other unit is mounted on themoving plane side of the stage.

Movement of the stages as described above generates reaction forces thatcan affect performance of the photolithography system. Reaction forcesgenerated by the wafer (substrate) stage motion can be mechanicallytransferred to the floor (ground) by use of a frame member as describedin U.S. Pat. No. 5,528,100 and Japanese Laid-Open Patent ApplicationPublication No. 8-136475. Additionally, reaction forces generated by thereticle (mask) stage motion can be mechanically transferred to the floor(ground) by use of a frame member as described in U.S. Pat. No.5,874,820 and Japanese Laid-Open Patent Application Publication No.8-330224. The disclosures of U.S. Pat. Nos. 5,528,100 and 5,874,820 andJapanese Paid-Open Patent Application Publication Nos. 8-136475 and8-330224 are incorporated herein by reference in their entireties.

The measurement system 22 monitors movement of the reticle 28 and thewafer 30 relative to the optical assembly 16 or some other reference.With this information, the control system 24 can control the reticlestage assembly 18 to precisely position the reticle 28 and the workpiece stage assembly 20 to precisely position the wafer 30. The designof the measurement system 22 can vary. For example, the measurementsystem 22 can utilize multiple laser interferometers, encoders, mirrors,and/or other measuring devices.

The control system 24 receives information from the measurement system22 and controls the stage assemblies 18, 20 to precisely position thereticle 28 and the wafer 30. Additionally, the control system 24 cancontrol the operation of the components of the environmental system 26.The control system 24 can include one or more processors and circuits.

The environmental system 26 controls the environment in a gap (notshown) between the optical assembly 16 and the wafer 30. The gapincludes an imaging field. The imaging field includes the area adjacentto the region of the wafer 30 that is being exposed and the area inwhich the beam of light energy travels between the optical assembly 16and the wafer 30. With this design, the environmental system 26 cancontrol the environment in the imaging field. The desired environmentcreated and/or controlled in the gap by the environmental system 26 canvary accordingly to the wafer 30 and the design of the rest of thecomponents of the lithography machine 10, including the illuminationsystem 14. For example, the desired controlled environment can be afluid such as water. Alternatively, the desired controlled environmentcan be another type of fluid such as a gas. In various embodiments, thegap may range from 0.1 mm to 10 mm in height between top surface of thewafer 30 and the last optical element of the optical assembly 16.

In one embodiment, the environmental system 26 fills the imaging fieldand the rest of the gap with an immersion fluid. The design of theenvironmental system 26 and the components of the environmental system26 can be varied. In different embodiments, the environmental system 26delivers and/or injects immersion fluid into the gap using spraynozzles, electro-kinetic sponges, porous materials, etc. and removes thefluid from the gap using vacuum pumps, sponges, and the like. The designof the environmental system 26 can vary. For example, it can inject theimmersion fluid at one or more locations at or near the gap. Further,the immersion fluid system can assist in removing and/or scavenging theimmersion fluid at one or more locations at or near the work piece 30,the gap and/or the edge of the optical assembly 16. For additionaldetails on various environmental systems, see U.S. provisional patentapplications 60/462,142 entitled “Immersion Lithography Fluid ControlSystem” filed on Apr. 9, 2003, 60/462,112 entitled “Vacuum Ring Systemand Wick Ring System for Immersion Lithography” filed on Apr. 10, 2003,60/500,312 entitled “Noiseless Fluid Recovery With Porous Material”filed on Sep. 3, 2003, and 60/541,329 entitled “Nozzle Design forImmersion Lithography” filed on Feb. 2, 2004, all incorporated byreference herein in their entireties.

Referring to FIG. 2, a cross section of a lithography machineillustrating one embodiment of the invention is shown. The lithographymachine 200 includes a optical assembly 16 and a stage assembly 202 thatincludes a wafer table 204 and a wafer stage 206. The wafer table 204 isconfigured to support a wafer 208 (or any other type of work piece)under the optical assembly 16. An environmental system 26, surroundingthe optical assembly 16, is used to supply and remove immersion fluid212 from the gap between the wafer 208 and the last optical element ofthe optical assembly 16. A work piece exchange system 216, including awafer loader 218 (i.e., a robot) and an alignment tool 220 (i.e., amicroscope and CCD camera), is configured to remove the wafer 208 on thewafer table 204 and replace it with a second wafer. This is typicallyaccomplished using the wafer loader 218 to lift and remove the wafer 208from the wafer table 204. Subsequently, the second wafer (not shown) isplaced onto the wafer chuck 218, aligned using the alignment tool 220,and then positioned under the optical assembly 16 on the wafer table204.

With this embodiment, the wafer stage 206 includes an immersion fluidcontainment system 214 that is configured to maintain the immersionfluid 212 in the gap adjacent to the last optical element of the opticalassembly 16 during wafer exchange. The immersion fluid containmentsystem 214 includes a pad 222 that is adjacent to the wafer table 204. Asupport member 224, provided between the pad 222 and the wafer stage206, is used to support the pad 222. The wafer table 204 has a flatupper surface that is coplanar with a surface of the wafer 208. The pad222 also has a flat upper surface that is coplanar with the uppersurface of the wafer table 204 and the wafer surface. The pad 222 isarranged adjacent to the wafer table 204 with a very small gap (e.g.,0.1-1.0 mm) so that the immersion fluid 212 is movable between the wafertable 204 and the pad 222 without leaking. During a wafer exchange, thewafer stage 206 is moved in the direction of arrow 226 so that the pad222 is positioned under the optical assembly 16 in place of the wafertable 204, maintaining the fluid in the gap or maintaining the size ofthe fluid gap. After the new wafer has been aligned, the wafer stage ismoved back to its original position so that the pad 222 is removed fromthe gap as the second wafer is positioned under the optical assembly 16.In various embodiments, the pad 222 is disposed continuously adjacent tothe wafer table 204 with no gap. Vertical position and/or tilt of thewafer table 204 can be adjusted so that the wafer table surface iscoplanar with the pad surface, before the wafer table 204 is moved outfrom under the optical assembly 16. Maintaining the gap between the pad222 and the optical assembly 16 is not limited to just a wafer exchangeoperation. The pad 222 can be large enough to maintain the immersionfluid 212 in the space between the pad 222 and the optical assembly 16during an alignment operation or a measurement operation. In thoseoperations, a part of the area occupied by the immersion fluid 212 maybe on the upper surface of the wafer table 204.

Referring to FIGS. 3A and 3B, a cross section and a top down view ofanother immersion lithography machine according to another embodiment ofthe present invention are shown. The lithography machine 300 includes anoptical assembly 16 and a stage assembly 302 that includes a wafer table304 and a wafer stage 306. The wafer table 304 is configured to supporta wafer 308 (or any other type of work piece) under the optical assembly16. An environmental system 26, surrounding the optical assembly 16, isused to supply and remove immersion fluid 312 from the gap between thewafer 308 and the lower most optical element of the optical assembly 16.A work piece exchange system 316, including a wafer loader 318 and analignment tool 320, is configured to remove the wafer 308 on the wafertable 304 and replace it with a second wafer. This is accomplished usingthe wafer loader 318 to remove the wafer 308 from the wafer table.Subsequently, the second wafer (not shown) is placed onto the waferchuck 318, aligned using the alignment tool 320, and then positionedunder the optical assembly 16. As best illustrated in FIG. 3B, a set ofmotors 322 are used to move the wafer assembly 302 including the wafertable 304 and wafer stage 306 in two degrees of freedom (X and Y) duringoperation. As noted above, the motors 322 can be any type of motors,such as linear motors, rotary motors, voice coil motors, etc.

The immersion lithography machine 300 also includes an immersion fluidcontainment system 324 that is configured to maintain the immersionfluid 312 in the space below the optical assembly 16 while the wafertable 304 is away from under the optical assembly. The immersion fluidcontainment system 324 includes a pad 326, a motor 328, and a controlsystem 330. The pad 326 can be positioned adjacent to the opticalassembly 16 and the wafer table 304. The wafer table 304 has a flatupper surface that is coplanar with a surface of the wafer 308. The pad326 has a flat upper surface that is coplanar with the upper surface ofthe wafer table 304 and the wafer surface. The pad 326 is movable in theX and Y directions using the motor 328, which is controlled by thecontrol system 330. The motor 328 can be any type of motor as well asthe motors 322. The pad 326 is positioned under the optical assembly 16when the wafer table 304 (the wafer stage 306) is away from under theoptical assembly 16. During a wafer exchange, the wafer table 304 movesaway from the optical assembly 16. Simultaneously, the control system330 directs the motor 328 to move pad 326 under the optical assembly 16,replacing the wafer table 304. The pad 326 thus retains the immersionfluid 312 within the gap under the optical assembly 16. After the newwafer has been aligned using the alignment tool 320, the wafer table 304is repositioned under the optical assembly 16. At the same time, thecontrol system 330 directs the motor 328 to retract the pad 326 from thegap, preventing the escape of the immersion fluid 312. In the waferexchange operation, the control system 330 moves the wafer table 304 andthe pad 326 with a small gap between the wafer table 304 and the pad326, while the immersion fluid 312 below the optical assembly 16 movesbetween the wafer table 304 and the pad 326. The immersion fluidcontainment system 324 thus maintains the immersion fluid 312 from thegap during wafer exchange. In this embodiment, the wafer table 304 (thewafer stage 306) and the pad 326 are movable separately. Therefore, thewafer table 304 is movable freely while the immersion fluid 312 ismaintained in the space between the pad 326 and the optical assembly 16.In various embodiments of the invention, the control system 330 may be aseparate control system or it can be integrated into the control systemused to control the motors 322 for positioning the wafer stage 306 andwafer table 304. Vertical position and/or tilt of at least one of thewafer table 304 and the pad 326 may be adjusted so that the wafer tablesurface is coplanar with the pad surface, before the wafer table ismoved out from under the optical assembly 16. The operation, in whichthe wafer table 304 is away from the optical assembly 16, is notnecessarily limited to a wafer exchange operation. For example, analignment operation, a measurement operation or other operation may beexecuted while maintaining the immersion fluid 312 in the space betweenthe pad 326 and the optical assembly 16.

Referring to FIGS. 4A and 4B, two cross sections of an immersionlithography machine are shown. The lithography machine 400 includes anoptical assembly 16 and a stage assembly 402 that includes a wafer table404 and a wafer stage 406. The wafer table 404 is configured to supporta wafer 408 (or any other type of work piece) under the optical assembly16. An environmental system 26 (410), surrounding the optical assembly16, is used to supply and remove immersion fluid 412 from the gapbetween the wafer 408 and the lower most optical element of the opticalassembly 16. A work piece exchange system 416, including a wafer loader418 and an alignment tool 420, is configured to remove the wafer 408 onthe wafer table 404 and replace it with a second wafer. This isaccomplished using the wafer loader 418 to remove the wafer 408 from thewafer table 404. Subsequently, the second wafer (not shown) is placedonto the wafer chuck 418, aligned using the alignment tool 420, and thenpositioned under the optical assembly 16 as illustrated in the FIG. 4A.

The immersion lithography machine 400 also includes an immersion fluidcontainment system 424 that is configured to maintain the immersionfluid 412 in the space below the optical assembly 16 while the wafertable 404 is away from under the optical assembly 16. The immersionfluid containment system 424 includes a pad 426, a first clamp 428provided on the optical assembly 16 and a second clamp 430 provided onthe wafer table 404. When the immersion fluid 412 is between the opticalassembly 16 and the wafer table 404 (or the wafer 408), the pad 426 isheld by the second clamp 430 in place on the wafer table 404. When thewafer table 404 is away from the optical assembly 16, for example duringa wafer exchange operation, the pad 426 is detached from the wafer table404 and held by the first clamp 428 to maintain the immersion fluid 412between the optical assembly 16 and the pad 426. The wafer table 404 hasa flat upper surface that is coplanar with a surface of the wafer 408.The pad 426 held on the wafer table 404 also has a flat upper surfacethat is coplanar with the upper surface of the wafer table 404 and thewafer surface. Therefore, the immersion pad 426 and wafer 408 can bemoved under the optical assembly without the immersion fluid leaking. Invarious embodiments, the clamps 428 and 430 can be vacuum clamps,magnetic, electro-static, or mechanical.

As best illustrated in FIG. 4A, the pad 426 is positioned on the wafertable 404 during exposure of the wafer 408. The second clamp 430 is usedto hold the pad 426 in place on the table 404 during the wafer exposure.During a wafer exchange as illustrated in FIG. 4B, the wafer table 404is moved in the direction of arrow 432 so that the pad 426 is positionedunder the optical assembly 16 in place of the wafer 408. When thisoccurs, the second clamp 430 holding the pad 426 to the wafer table 404is released while first clamp 428 clamps the pad 426 to the opticalassembly 16. As a result, the immersion fluid 412 is maintained underthe optical assembly while the wafer 408 is exchanged. After the newwafer has been aligned, the wafer table 404 is moved in the directionopposite arrow 432 so that the new wafer is positioned under the opticalassembly. Prior to this motion, the first clamp 428 is released whilethe second clamp 430 again clamps the pad 426 to the wafer table 404. Inthis embodiment, the wafer table 404 is freely movable while the pad 426is clamped by the first clamp 428.

In various embodiments, the operation, in which the pad 426 is clampedby the first clamp 428, is not limited to only a wafer exchangeoperation. An alignment operation, a measurement operation, or any otheroperation can be executed while the immersion fluid 412 is maintained inthe space between the optical assembly 16 and the pad 426 clamped by thefirst clamp 428. Also, the clamp 428 can be provided on the frame 12 orother support member, and the clamp 430 can be provided on the waferstage 406. The pad 426 can be held on a movable member other than thestage assembly 402.

FIGS. 5A and 5B are top down views of two different twin stage immersionlithography systems according to other embodiments of the presentinvention. For the basic structure and operation of the twin stagelithography systems, see U.S. Pat. No. 6,262,796 and U.S. Pat. No.6,341,007. The disclosures of U.S. Pat. No. 6,262,796 and U.S. Pat. No.6,341,007 are incorporated herein by reference in their entireties. Inboth embodiments, a pair of wafer stages WS1 and WS2 are shown. Motors502 are used to move or position the two stages WS1 and WS2 in thehorizontal direction (in the drawings), whereas motors 504 are used tomove or position the stages WS1 and WS2 in the vertical direction (inthe drawings). The motors 502 and 504 are used to alternatively positionone stage under the optical assembly 16 while a wafer exchange andalignment is performed on the other stage. When the exposure of thewafer under the optical assembly 16 is complete, then the two stages areswapped and the above process is repeated. With either configuration,the various embodiments of the invention for maintaining immersion fluidin the gap under the optical assembly 16 as described and illustratedabove with regard to FIGS. 2 through 4, can be used with either twinstage arrangement. With regard the embodiment of FIG. 2 for example,each wafer stage SW1 and SW2 of either FIG. 5A or 5B can be modified toinclude a pad 222 and a support member 224. With regard to theembodiment of FIG. 3, a single pad 326, motor 328, and control system330 could be used adjacent to the optical assembly 16. The pad 326 ismovable separately from the stages SW1 and SW2. During the time whenstages SW1 and SW2 are to be swapped, the pad 326 is moved to under theoptical assembly 16 to maintain the immersion fluid 312 below theoptical assembly 16. Finally with the embodiment of FIG. 4, a detachablesingle pad can be used. During the time when stages SW1 and SW2 are tobe swapped, the pad 426 is used to maintain the immersion fluid in thegap as illustrated in FIG. 4B. On the other hand during exposure, thepad is clamped onto the wafer table on the wafer stage that is beingexposed. In this manner, only a single pad is needed for the two stagesWS1 and WS2. Alternatively, as described below, the second stage canalso be used as the pad.

Referring to FIG. 6A, a top down view of a twin stage lithographymachine illustrating one embodiment of practicing the invention isshown. In this embodiment, the immersion lithography system 600 includesfirst stage 604 and second stage 606. The two stages are moved in the Xand Y directions by motors 602. In this embodiment, the stages 604 and606 themselves are used to contain the immersion fluid in the gap. Forexample as shown in the Figure, the first stage 604 is positioned underthe optical assembly 16. When it is time for the work piece to beexchanged, the motors 602 are used to position the second stage 606 witha second work piece adjacent to the first stage 604. With the two stagespositioned side-by-side, they substantially form a continuous surface.The motors 602 are then used to move the two stages in unison so thatthe second stage 604 is position under the optical assembly 16 and thefirst stage is no longer under the optical assembly 16. Thus when thefirst work piece is moved away from the optical assembly 16, theimmersion fluid in the gap is maintained by the second stage 606, whichforms the substantially continuous surface with the first stage. Invarious alternative embodiments, the second stage 606 could also be a“pad” stage that contains a pad that is used to maintain the immersionliquid in the gap while a second work piece is being placed onto thefirst stage 604. Similarly, the motor arrangement shown in either FIG.5A or 5B could be used.

Referring to FIGS. 6B-6E, a series of diagrams illustrating a work pieceexchange according to one embodiment of the invention is illustrated.FIG. 6B shows a wafer on stage 604 after exposure is completed. FIG. 6Cshows the second stage 606 in contact (or immediately adjacent) with thefirst stage 604 under the optical assembly 16. FIG. 6C shows a transfertaking place, i.e., the second stage 606 is positioned under the opticalassembly 16. Finally, in FIG. 6E, the first stage 604 is moved away fromthe optical assembly 16. As best illustrated in FIGS. 6C and 6D, the twostages 604 and 606 provide a continuous surface under the opticalassembly 16 during a transfer, thus maintaining the immersion fluid inthe gap. In the embodiment shown, the second stage 606 is a pad stage.This stage, however, could also be a work piece stage as noted above.

In the various embodiments described above, the pad can be made of anumber of different materials, such as ceramic, metal, plastic. Thesematerials may also be coated with Teflon according to other embodiments.The size of the pad also should be sufficient to cover the area occupiedby the immersion fluid. In the various embodiments described above, thesurface of the last optical element of the optical assembly 16 isconstantly under immersion fluid environment, preventing the formationof a fluid mark (e.g. “a water mark”).

Semiconductor wafers can be fabricated using the above describedsystems, by the process shown generally in FIG. 7A. In step 701 the workpiece's function and performance characteristics are designed. Next, instep 702, a mask (reticle) having a pattern is designed according to theprevious designing step, and in a parallel step 703 a wafer is made froma silicon material. The mask pattern designed in step 702 is exposedonto the wafer from step 703 in step 704 by a photolithography systemdescribed hereinabove in accordance with the invention. In step 705 thesemiconductor work piece is assembled (including the dicing process,bonding process and packaging process); finally, the work piece is theninspected in step 706.

FIG. 7B illustrates a detailed flowchart example of the above-mentionedstep 704 in the case of fabricating semiconductor work pieces. In FIG.7B, in step 711 (oxidation step), the wafer surface is oxidized. In step712 (CVD step), an insulation film is formed on the wafer surface. Instep 713 (electrode formation step), electrodes are formed on the waferby vapor deposition. In step 714 (ion implantation step), ions areimplanted in the wafer. The above mentioned steps 711-714 form thepreprocessing steps for wafers during wafer processing, and selection ismade at each step according to processing requirements.

At each stage of wafer processing, when the above-mentionedpreprocessing steps have been completed, the following post-processingsteps are implemented. During post-processing, first, in step 715(photoresist formation step), photoresist is applied to a wafer. Next,in step 716 (exposure step), the above-mentioned exposure work piece isused to transfer the circuit pattern of a mask (reticle) to a wafer.Then in step 717 (developing step), the exposed wafer is developed, andin step 718 (etching step), parts other than residual photoresist(exposed material surface) are removed by etching. In step 719(photoresist removal step), unnecessary photoresist remaining afteretching is removed.

Multiple circuit patterns are formed by repetition of thesepreprocessing and post-processing steps.

While the particular lithography machines as shown and disclosed hereinare fully capable of obtaining the objects and providing the advantagesherein before stated, it is to be understood that they are merelyillustrative embodiments of the invention, and that the invention is notlimited to these embodiments.

What is claimed is:
 1. A lithographic projection apparatus comprising: asubstrate table on which a substrate is held; a projection system thatprojects a patterned beam onto a target portion of the substrate held bythe substrate table, an immersion liquid, through which the beam is tobe projected, being in a space between the projection system and thesubstrate; and a flat pad that keeps the projection system in contactwith the immersion liquid when the substrate, at least while held on thesubstrate table, comes out of contact with the immersion liquid, theflat pad being separate from the substrate table.
 2. An apparatusaccording to claim 1, wherein the flat pad is positionable opposite theprojection system such that the immersion liquid can be confined betweenthe projection system and the flat pad.
 3. An apparatus according toclaim 2, further comprising an attachment device that releasablyattaches the flat pad over a final optical element of the projectionsystem.
 4. An apparatus according to claim 3, wherein the attachmentdevice comprises a vacuum outlet that attracts the flat pad toward theprojection system.
 5. An apparatus according to claim 2, wherein thesubstrate table comprises a holding device that releasably holds theflat pad to the substrate table.
 6. An apparatus according to claim 2,wherein, in a stowed position, the flat pad has a primary surfacesubstantially co-planar with a surface of the substrate facing theprojection system.
 7. An apparatus according to claim 2, wherein thelithographic apparatus comprises a controller that moves the substratetable relative to the projection system so that the flat pad confinesthe immersion liquid between the flat pad and the projection system. 8.An apparatus according to claim 1, wherein the flat pad has a primarysurface substantially co-planar with a surface of the substrate facingthe projection system and is closely adjacent to an edge of thesubstrate.
 9. An apparatus according to claim 1, further comprising astructure extending along at least a part of a boundary of the spacebetween the projection system and the substrate table and wherein theflat pad, when keeping the projection system in contact with theimmersion liquid, is positioned to cover the structure.
 10. An apparatusaccording to claim 1, wherein the flat pad keeps the projection systemin contact with the immersion liquid when the substrate is moved awayfrom under the projection system.
 11. A device manufacturing methodcomprising: providing an immersion liquid to a space between aprojection system and a substrate; projecting a patterned beam ofradiation, through the immersion liquid, onto a target portion of thesubstrate using the projection system; and maintaining the projectionsystem in contact with the immersion liquid when the substrate, at leastwhile on a substrate table, comes out of contact with the immersionliquid by placing a flat pad, which is separate from the substratetable, adjacent to the projection system in place of the substrate. 12.A method according to claim 11, wherein maintaining the projectionsystem in contact with liquid comprises positioning the flat pad on aside opposite the projection system such that liquid is confined betweenthe projection system and the flat pad.
 13. A method according to claim12, further comprising releasably attaching the flat pad over a finaloptical element of the projection system.
 14. A method according toclaim 13, wherein releasably attaching comprises attaching the flat padusing vacuum.
 15. A method according to claim 12, wherein the flat pad,when keeping the projection system in contact with the immersion liquid,is positioned to cover a structure that extends along at least a part ofa boundary of the space between the projection system and the substratetable.
 16. A method according to claim 12, comprising moving thesubstrate table relative to the projection system so that the flat padconfines the immersion liquid between the flat pad and the projectionsystem.
 17. A method according to claim 11, comprising providing theimmersion liquid to a space between a final lens of the projectionsystem and the substrate.
 18. A method according to claim 11, whereinthe maintaining comprises maintaining the projection system in contactwith the immersion liquid after the substrate has been moved away fromunder the projection system.